It is of great interest to be able to detect and image leakage of widely used industrial gases such as, for example, sulphur hexafloride gas (SF6) and gaseous hydrocarbons. For example small but cumulatively significant releases of methane, as from oil rigs, tank farms, pipelines and compression stations constitute an economical loss for petrochemical industries, an environmental problem and a safety concern. Releases in connection with accidents involving transport vehicles or in industrial operations and land fill garbage dump sites need to be identified. In the electrical energy generation and distribution industries, pressurized SF6 gas is widely relied upon as an insulating coolant in electrical apparatus including power transformers. Leakage is a random but chronic problem. If an overheating problem is undetected expensive equipment failure may follow, and if detected, at least the coolant needs replenishing. A minor leak may discharge only a fraction of a gram per minute and hence is difficult to detect and localize. Over a period of a few years such minor leakage may add up to many tens or even hundreds of kilograms, necessitating the expense of replacement SF6 gas and the time of skilled technicians in maintenance activities that may have otherwise been avoidable if the leakage was readily identifiable and hence remedied. Among the physical characteristics of most gases are those of line spectral absorbtion and emission of electromagnetic energy. Most gases are unique in this characteristic, as with respect to other gases. In other words a gas will exhibit an absorption of energy of a particular wavelength, or possibly of several different wavelengths, somewhere within in a spectrum of a range of deep infrared though high ultra violet, while energies of other wavelengths pass through the gas unimpeded. For example, SF6 exhibits an intense absorption characteristic at 10.6 micrometers. Methane exhibits an intense absorption characteristic at 7.7 micrometers and a less intense but significant absorption characteristic at 3.2 micrometers. Gas imaging is performed based on the wavelength absorption characteristic of a gas of interest in the electromagnetic spectrum including ultraviolet through infrared. Active gas imaging techniques use an artificial light source such as a scanning laser, tuned to the absorption wavelength of the gas, in contrast with passive gas imaging techniques which use natural light and background electromagnetic radiation. It is, therefore, desirable to image and localize a fugitive gas leakage by using a portable apparatus which is functional, while an operator of the apparatus is somewhat remote from the source of the leak. A portable and economical imaging device would expedited appropriate repair of apparatus showing gas leaks.
Active gas imaging tends to be cumbersome and expensive. One example of an active gas imaging technique is disclosed by McRae et al. in U.S. Pat. No. 4,555,627, issued on Nov. 26, 1985. McRae et al. disclose a video imaging system for detecting gas leaks. Visual displays of invisible gas clouds are produced by radiation augmentation of the field of view of an imaging device by radiation corresponding to an absorption line of the gas to be detected. The field of view of an imager is irradiated by a scanning laser beam. When a detectable gas is present, back-scattered laser beam is highly attenuated, producing an image with a region of contrast.
Passive gas imaging although less expensive than active gas imaging tends to be less sensitive. An example is disclosed in U.S. Patent Application 20030025081 filed Feb. 6, 2003, to Edner et al. wherein a passive gas imaging technique is disclosed. Edner et al. teach a method for imaging of gas emissions utilizing optical techniques combining gas correlation techniques with thermal background radiation or self-emission radiation. A simultaneous recording of images with and without filtering through a gas-filled cell is utilized for the identification of a selected gas.
Both gas imaging techniques have substantial drawbacks and performance limits. The active gas imaging techniques requires a laser source and complex associated equipment which limits their applications in the field where portability is required. The passive gas imaging technique employs a gas cell within a cumbersome optical array and is generally less sensitive. Both techniques tend to be limited to operation in relatively close proximity to the suspected gas leak and detection of only higher volumes of gas.